1. Field of the Invention
Present invention pertains generally to measuring and testing devices and more particularly to an in-circuit test device for testing transistors.
2. Description of the Background
Quality control plays an important role in present day manufacturing and production techniques. A key factor in controlling the quality of electronic products is the ability to test component systems prior to final assembly of the device. Loaded printed circuit boards have been tested in a number of different ways in the past. One method comprises Functional Testing in which signals are applied to the input of the board and the output of the board is monitored and compared to desired results to determine if the board is functioning properly. The simplest implementations of this technique merely provide a pass/fail result for each particular board tested with no indication of the identity or location of malfunctioning components in the board. Failure rates of the boards are normally much too high to rely upon Functional Testing as the only method of testing PC boards prior to assembly of the product. More complex Functional Testing Systems provide more information as to the identity and location of malfunctioning components on the PC board and have been used with some success as singular testing systems. These more complex Functional Testing Systems attempt to locate non-functioning components in the PC board by carefully analyzing the output of a non-functioning board for a series of specially programmed input signals that are designed to produce outputs which differ for each component, or series of components, that are defective. Of course, these more complex systems are difficult to produce because of the high degree of complexity of programming input signals and analyzing output data to determine the identity and location of one or more improperly functioning components, especially when large and more complex boards are being tested. Moreover, these tests, due to their complexity, often cannot provide sufficient information as to the location of non-functioning components so that further testing is required to prevent a high scrap rate of the PC boards. For these reasons, in-circuit testing techniques have been developed that individually test components which have been mounted on the PC board. In-circuit testing of a PC board, in this manner, clearly identifies the location of non-functioning components.
A number of problems, however, exist using in-circuit testing techniques. For example, since a large number of inter-connecting conductive paths are present between the components mounted on a PC board under test, various feedback paths are created.
A basis for testing all components using in-circuit test techniques is that a standardized test procedure be used for corresponding components on different boards. If the test must be customized each time a corresponding component is tested, such as by requiring different threshold values of current or voltage, the test cannot be easily automated to provide reliable results. A typical problem which exists in testing transistors is that the beta of the transistor can vary up to an order of magnitude. Some prior art devices have typically applied a constant base current and measured collector current to determine the beta of the transistor. If the beta of the transistor is high, a large current is drawn through the collector which decreases the collector/emitter voltage drop to the saturation voltage of the transistor. A typical circuit implementing these prior art techniques is disclosed in the 3253A Operating and Service Manual, page 3-11, section 320, Hewlett-Packard part #03253-90001, available from Manufacturing Test Division, Hewlett-Packard Company, P.O. Box 301, Loveland, CO 80539, which discloses operation and programming of the Model 3253A Analog In-Circuit Test Device for testing the beta of a transistor. When the transistor is in saturation, the beta of the transistor cannot be tested. To prevent the transistor from going into saturation the standardized base current must be reduced. Hence, the test fails as an automated procedure for testing the beta of the transistor when a high beta transistor is encountered. These problems cannot be readily overcome by providing a higher collector/emitter voltage drop since, as set forth above, a greater voltage drop will cause other active components on the board to be activated. Also, since the beta of the transistor in some prior art testing techniques has been computed using an a.c. signal, capacitive feedback loops often cause erroneous gain calculations.